a) Field of the invention
The present invention relates to a Schwarzschild optical system which is to be applied as an objective optical system for X-ray microscopes utilizing wavelengths within the soft X-ray zone.
b) Description of the prior art
Optical microscopes are conventionally used as instruments for observing small objects. In order to obtain an instrument which has higher resolution for observing smaller objects, it is desirable to use an objective lens system having a numerical aperture as large as possible since the resolution of the observing instrument is limited by the wavelength of the light to be utilized for observation and the numerical aperture of the objective lens system used therein (the so-called diffraction limit). However, since the numerical aperture of the objective lens system can be enlarged only within a certain limit, there an inevitable tendency, for observing smaller objects, to shorten the wavelength of the light to be utilized for observation.
Under the recent circumstances where X-ray sources of relatively good qualities are available and X-ray microscopes are developed for satisfying the increasing desire to observe smaller objects, there is produced a demand for objective lens systems which have good performance for imaging the X-ray.
As an objective optical system which is usable for imaging the X-ray, there is conventionally known the Schwarzschild optical system. This optical system consists, as shown in FIG. 1, of a large concave mirror A which has a spherical surface having an opening formed at the center thereof and a small convex mirror B which has a spherical surface arranged opposite to the opening of the concave mirror A so that a light beam emitted from an object point O is reflected by the concave mirror A and the convex mirror B in this order, and then is focused onto an image point I. (This objective optical system may be used in such a position that the image point I and the object point O are replaced with each other.) The X-ray microscopes which use the optical system described above as an objective optical system are classified into an imaging type microscope and a scanning type microscope. The scanning type microscope comprises, as shown in FIG. 2, an X-ray source 1, a pinhole 2, an objective optical system 3, a sample 4 which is arranged to be freely movable in the direction perpendicular to the optical axis, and an X-ray detector 5; these members being coaxially arranged. The scanning type X-ray microscope is constructed to detect an image of the sample 4 while condensing the X-ray beam having passed through the pinhole 2 onto the sample 4 as a small spot and scanning a predetermined range of the sample 4 by moving the sample 4 on a plane perpendicular to the optical axis.
On the other hand, the imaging type X-ray microscope comprises, as shown in FIG. 3, an X-ray source 1, a condenser lens 6, a sample 4, an objective optical system 3 and a detector 5 which are coaxially arranged. An X-ray beam emitted from the X-ray source 1 is focused by the condenser lens 6 as a spot having a certain size on a predetermined area on the sample 4. The X-ray beam is transmitted through or is diffracted by the sample 4, and then is focused by the objective optical system 3 to form a magnified image of the sample 4 on the detector 5.
As for correction of aberrations in these objective optical systems, it is sufficient to correct aberrations only within a narrow zone in the vicinity of the optical axis for the objective optical system used in the scanning type microscope in which the sample is moved, whereas aberrations must be corrected within a relatively broad range up to a certain image height in the objective optical system arranged in the imaging type microscope.
Accordingly, the requirements to be satisfied by objective lens systems to be used for the X-ray microscopes or the key points for designing these objective optical systems can be summarized as follows:
(1) Aberrations are corrected favorably; PA0 (2) The objective optical systems have large numerical apertures; and PA0 (3) Optical performance of the objective optical systems is not adversely affected by misalignment of the mirrors.
For satisfying the requirement (3) out of the requirements mentioned above, it is more advantageous to select a non-concentric type Schwarzschild optical system in which the center of curvature of the concave mirror is not coincident with that of the convex mirror since this type of objective optical system has performance less affected by the misalignment of the mirrors.
As the non-concentric type Schwarzschild optical system, there are conventionally known the optical systems proposed by I. Lovas (High Resolution Soft X-ray Optics, SPIE vol. 316 (1981)) and those proposed by J. A. Trail (SPIE vol. 563 (1985) p90). When the departure between the centers of curvature of the two mirrors is expressed as a difference between a distance W2 from the object point 0 to the center of curvature C2 of the concave mirror and a distance W1 from the object point 0 to the center of curvature C1 of the convex mirror which is in a form normalized to focal length f of the objective lens system as a whole, i.e., (W2-W1)/f (this value is referred to as departure between centers DC), the objective optical systems desclosed by the above-mentioned SPIE vol. 316 have DC's of -0.022 to -0.07 and a numerical aperture NA of 0.2 on the object side (on the shorter conjugate side), whereas the objective optical systems disclosed by the above-mentioned SPIE vol. 563 have DC's of the order of 0.06 and numerical apertures NA's of 0.2, 0.3 and 0.4 on the object side.
A larger departure between centers DC is more desirable for lessening the influence due to the misalignment of the mirrors, but too large a departure between centers DC makes it more difficult to correct aberrations. It is therefore necessary to determine design parameters for an objective optical system while considering good balance between the influence due to misalignment of the mirrors and the correction of aberrations. Though each of the two mirrors is designed as a spherical mirror in the conventional Schwarzschild optical systems, aberrations cannot be corrected sufficiently only with spherical mirrors in Schwarzschild optical systems which have large numerical apertures and large departures between centers.